This invention relates to an image recording method capable of producing a high-quality image by scatter recording the image in an auxiliary scanning direction using an apparatus which typically performs image recording by scanning a recording medium in the main scanning direction as it is transported in the auxiliary scanning direction.
Image recording apparatus in common use today are of such a type that a thermal head, laser beam and other suitable means are employed to perform image recording on a medium. The thermal head is typically in the form of a linear arrangement of many heating elements and a thermal recording material as the recording medium is compressed against the thermal head and transported in a direction generally perpendicular to the direction of the arrangement of the heating elements as they are controlled individually in accordance with the image data, whereby the desired two-dimensional toned image is recorded.
The toned images are formed in the following way. The image with data D.sub.p =1 which is to be subjected to recording is formed by heating the heating elements for t seconds. The image with data D.sub.p =2 which is also to be subjected to recording is formed by heating for 2t seconds and similarly the images with data D.sub.p =3, 4 and 5 which are to be subjected to recording are formed by heating for 3t, 4t and 5t seconds, respectively. As a result, pixels are formed on the recording medium, producing a color with differing areas in accordance with the data being subjected to recording over the range of one-pixel width in the direction of transport, whereby the desired toned image is recorded. This is the method of recording toned images by pulse-width modulation and substantially the same procedure may be followed in pulse-numbers modulation.
In this method of recording toned images, pixels are always recorded from a fixed point on one side of a single pixel width in the direction of transport, namely, from the record start position, and no record is made at points on the other side, namely, on the record end position side; hence, image recording is localized on the record start position side. This produces a two-dimensional image which, when viewed as a whole, has a pronounced degree of graininess.
To deal with this difficulty, the present inventors previously filed Unexamined Published Japanese Patent Application No. 96625/1995 and proposed an image recording method and apparatus, in which the image data of an individual image composing pixel is divided into generally equal portions of image data and the thus divided image data are recorded by image recording means using a thermal head, a laser beam or the like as they are scattered in the direction of transport of the recording medium (which is transported in the auxiliary scanning direction). By employing the proposed method and apparatus, localization of image recording within a single pixel width in the direction of transport is avoided, thereby enabling the formation of high-quality images which have no graininess.
When image data is recorded with it being scattered as generally equal portions of image data, the localization of image recording is avoided to eliminate image graininess and, images of particularly high quality are obtained if their density is higher than a certain level. However, image data scattering is also applied to the low-density areas of the image to be recorded; therefore, if the image data is subjected to pulse-width modulation or pulse-numbers modulation for recording with the thermal head, the duration of heating time which is already narrow at low density is further divided into equal portions and the temperature of the thermal head, or its peak temperature, is not sufficiently increased to perform the intended thermal recording and the transportability of the recording medium is deteriorated and the image being produced becomes accordingly scratchy or its quality is deteriorated.
On the other hand, if thermal recording is performed in the usual manner without dividing the image data, the peak temperature of the thermal head becomes so much elevated when recording the high-density areas of the image that the durability of the thermal head is deteriorated, or its use life is shortened. The same problem has occurred in image recording with a laser beam and an elevated peak temperature in the high-density areas of the image being recorded is damaging to the surface of the recording medium.